Material flow patterns and cavity model in friction-stir welding of aluminum alloys
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1/17/04
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Material Flow Patterns and Cavity Model in Friction-Stir Welding of Aluminum Alloys LIMING KE, LI XING, and J.E. INDACOCHEA Friction-stir welding (FSW) of 3-mm-thick plates of 6061 Al and LF6 Al was conducted and the materials’ flow patterns in the weld nugget along three perpendicular planes were analyzed. The onion structure viewed on any cross section normal to the travel direction is independent of weld position. The weld morphology was examined along its length by considering planes of different depths parallel to the surface. These showed semicircle streaks whose shapes depended on the depth of the observation plane. It is determined that the weld nugget is composed of a series of identical half ellipsoid regions. A tentative simplified cavity model is presented to explain the mass flow pattern and formation of defects in the weld nugget. This model is based on the assumption that only the metal between the pin surface and the last maximum circle created by the pin rotation is in a plasticized state. From this model, it is shown that the location and size of the cavity formed during the rotation of the pin changes cyclically and it is related to the position of the pin’s center. The holes or slots left in the weld nugget center or near the advancing side are directly related to the size of the cavity. The welding parameters or weld pitch affects the volume of the cavity, and consequently influence the weld defects. A large weld pitch will cause holes to be formed in the weld nugget because of the large cavity. The flow patterns, which show that the plasticized material flows from both advancing and retreating sides to the weld center behind the pin, can be easily explained with this cavity model.
I. INTRODUCTION
FRICTION-stir welding (FSW) is a rather new joining technology invented by The Welding Institute in 1991.[1] In this process, a shouldered pin tool is rotated around its axial and plunged into the weld interface until the tool’s shoulder is pressed against the surface of the workpiece materials. The friction heat, mainly generated by the contact between the shoulder and the workpiece materials, heats the metal around the pin. The weld metal is not melted but it becomes softened by the heat, and the metal plastically flows from the front of the welding tool to the rear of the tool and forms a sound weld nugget under the tool’s pressure.[2] This joining procedure can be used in welding metals that were thought unweldable or difficult to weld by conventional fusion welding. The welding defects usually related to fusion welding do not appear in FSW and, most important, the distortion is usually small. This technique has found several applications in shipbuilding and in the space industry, where welding of aluminum alloys is considerable.[3,4] Further applications of this welding process can also be found in the ocean, transportation, automobile, and electrical industries. Although extensive work has been done on FSW,[5,6] microstructure, properties, and metal flow behavior o
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